---
title: "Retrofit versus repair; column jacketing"
subject: "Building Technology"
chapter: 11
key: "semester_6/building_technology:retrofit-vs-repair-jacketing"
source: https://www.bctengineer.com/q/semester_6/building_technology/retrofit-vs-repair-jacketing
site: BCT Engineering
---

# Retrofit versus repair; column jacketing

Building Technology · Chapter 11: Earthquake protection & Retrofitting in building

- Asked in: 2078 Poush, 2+3 marks
- Video: [RETROFITTING (REPAIR AND RESTRENGTHNING) TECHNIQUE:JACKETING METHOD](https://www.youtube.com/watch?v=Lm-YADUnd_0) — CivilTech
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/retrofit-vs-repair-jacketing
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-11

## Question

How does retrofitting differ from repair? Write a note on jacketing of column for retrofitting.

## Short answer

Repair restores a damaged building to its original condition, while retrofitting upgrades its seismic capacity beyond the original. Column jacketing for retrofitting involves adding a reinforced concrete, steel, or FRP layer around the column to increase strength and ductility, with new vertical bars and closely spaced ties anchored to the footing and slabs.

## Answer

#### Retrofit versus repair
| Basis | Repair | Retrofit |
|---|---|---|
| Aim | Restore damaged building to its original state | Upgrade capacity to resist future earthquakes |
| Strength after work | Same as before damage | Greater than original |
| Applied to | Damaged buildings | Damaged or undamaged, deficient buildings |
| Seismic resistance | Not improved | Improved |
| Examples | Filling cracks, plastering, replacing broken bricks | Jacketing, shear walls, bracing, FRP |

#### Jacketing of columns
**Column jacketing** increases the strength, ductility and confinement of weak columns by adding a layer of material around them.
- **RCC jacket:** after roughening the column surface, new vertical bars and closely spaced ties (6-8 mm at 75-100 mm c/c) are placed, dowelled to the footing and slabs, and concrete (min. 75-100 mm thick) is cast with a bonding agent.
- **Steel jacket:** steel plates or angles with battens are placed around the column and the gap is grouted.
- **FRP wrap:** carbon/glass fibre wrapped around the column.

```text
 +-----------+
 | +-------+ |
 | |column | |  <- new bars + ties
 | +-------+ |
 +-----------+
```
It increases axial and shear capacity, but increases the column size.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does repair differ from retrofit?
   A. Repair restores the original state; retrofit upgrades capacity beyond the original
   B. Repair upgrades capacity; retrofit only restores
   C. Both give identical strength
   D. Retrofit is applied only to damaged buildings
   Answer: A. Repair brings back the old strength while retrofit improves seismic resistance. (Key point: Repair vs retrofit aim)

2. Retrofit can be applied to:
   A. Only damaged buildings
   B. Damaged or undamaged deficient buildings
   C. Only new buildings
   D. Only heritage buildings
   Answer: B. Repair is for damaged buildings; retrofit can also be applied to undamaged deficient ones. (Key point: Applicability)

3. Which describes an RCC column jacket?
   A. Plain concrete poured without roughening
   B. Only paint on the column
   C. Roughened surface, new bars and closely spaced ties dowelled to footing/slabs, concrete at least 75-100 mm thick
   D. Ties spaced at 500 mm
   Answer: C. Surface roughening and bonding agent give composite action; closely spaced ties confine the column. (Key point: RCC jacket)

4. What is the main drawback of column jacketing?
   A. It reduces axial capacity
   B. It reduces confinement
   C. It reduces ductility
   D. It increases the column size
   Answer: D. Jacketing raises axial and shear capacity, ductility and confinement but enlarges the section. (Key point: Jacketing effect)

## Related in Building Technology

- [Repair versus retrofit; configuration factors](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-vs-repair-config) — Building Technology, chapter 11
- [Retrofitting and jacketing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-jacketing) — Building Technology, chapter 11, asked 2×
- [Conventional strengthening and innovative-material retrofit](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-strengthening-innovative) — Building Technology, chapter 11
- [Retrofitting a brick masonry building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-brick-masonry) — Building Technology, chapter 11